Process for heat-treating a bearing alloy



PROCESS FOR HEAT-TREATING A BEARING ALLOY Erich Pelzel, Stolberg,Germany, assignor to Vereinigte Osterreichische Eiseuund StahlwerkeAktiengesellscbaft, Linz (Danube), Austria, :1 firm No Drawing.Application July 5, 1956 Serial No. 595,835

Claims priority, application Austria February 9, 1952 2 Claims. (Cl.148-13) The invention relates to an aluminum-zinc-copper alloy, which isparticularly suitable for replacing the expensive bronzes containingcopper and Zinc which have previously been used in most cases as abearing alloy. The invention relates also to a process for heat-treatingthe bearing members made of that alloy.

The present application is a continuation-in-part of the copending US.patent application Ser. No. 334,973 filed.February 3, 1953, nowabandoned. 7

Many alloys are known which consist of aluminum and copper, balancezinc. Previously, however, such alloys have been used only in singlecases in bearings. For this purpose not only the generally desiredhigher characteristics as regards tensile strength, elongation andBrinell hardness are to be taken into consideration but also theinternal structure and the adherence of oil films on the bearingsurface. In that respect too bronzes are preferable to alloys of zinc,aluminum and copper.

In the co-pending application mentioned hereinbefore, an alloy wasproposed for bearing purposes which contains 30-66% of aluminum, copperto the amount of at least one sixth and at most one fourth of the actualaluminum content, and the balance of zinc.

The general technological properties of such alloys are satisfactory butit has been found in experiments that in certain marginal regions of thealuminum content disadvantages occur which cannot be tolerated where thealloy is used for bearing bushings.

It has been found that the disintegration of the structure and resultantchanges in volume increase as the content of aluminum is reduced. Forinstance, in an alloy consisting of 30% aluminum, copper, balance zinc,a dimensional change of 01-02% will result after a storage time of onemonth. Such a change is not tolerable for bearing bushings because sucha shrinkage will obstruct the running of the shaft. Tests with alloyshaving a higher content of aluminum have shown that these detrimentalshrinkage properties do no longer occur in alloys with 35% aluminum,6.5% copper and 58.5% zinc. As a result, it is apparent that thealuminum content should not be less than 35%.

In further tests it has been shown that the upper limit of the aluminumcontent is also significant for bearing alloys because this upper limitis of special influence on the structure of the bearing metal. In thatrespect the interrelations are of a somewhat complex nature, as willbecome apparent from the following discussion.

When the alloys are being processed with the usual melting and pouringequipment a certain contamination of the alloy by iron cannot beavoided. That contamination has proved to be tolerable when the ironcontent of low-aluminum alloys does not exceed 0.3%. photographs of suchalloys reveal precipitates consisting of crystals of iron and zinc inthe form of small needle which are not disturbing.

With higher contents of aluminum, particularly up to the limit of 66%stated in the co-pending application Micro 0 Elongation: 3-6% mentionedhereinbefore, it was found that the precipitation of iron and zinccrystals increases disproportionately, i.e., by a multiple amount,compared to alloys having a low aluminum content. for this phenomenonresides in that with a high aluminum content a ternary field is enteredwherein complex aluminum and iron silicon compounds are formed.

These diflferences depending on the aluminum content appear to be due tothe following facts. Where the aluminum content and consequently alsothe silicon content is small, the afiinity of iron and zinc prevails sothat the said fine needlelike iron and zinc crystals are formed, whereinone part of iron combines with about 9 parts of zinc. having a highsilicon content, the afiinity relations are changed. One part of ironcombines with about twice that amount of silicon and seven times thatamount of aluminum to form a ternary crystal, which is voluminous owingto the high share of the lightweight aluminum. That process is promotedby the fact that the' rate of crystallisation of that ternary phase isso high that a relatively small number of coarse crystals areforrnedinstead of the aforementioned numerous fine needles.

The commercially pure aluminum, which is to be used for foundry purposesfor economic reasons, contains always impurities up to 0.5% silicon. Forthis reason the detrimental content of silicon in the alloy can be keptdown only by preventing the content of aluminum for exceeding atolerable amount.

The tests made have shown that the undesired coarse ternary crystalsbegin to occur with an aluminum content of 50%, i.e. with a siliconcontent of about 0.25% if the contamination of the aluminum by siliconamounts to 0.5 In order to avoid the disadvantages set forthhereinbefore, residing in undesired dimensional changes and theformation of coarse ternary crystals, therefore, the teaching given inthe co-pending application mentioned hereinbefore must be modified sothat the aluminum content of the bearing alloy amounts to 35-60%, thecopper content amounts at least to one sixth and at most to one fourthof the actual aluminum content, and the balance consists of zinc.

The range of alloys possible is limited by the series of alloys havingthe lowest copper content (copper content=% aluminum) and by the seriesof alloys having the highest copper content (copper content= A aluminumcontent).

The series of alloys having the lowest copper content Al Cu(=}6 Al) Zncum The series of alloys having the highest tent is:

copper COD- Al Cu(= )6 Al) Zn Specific gravity: 4.5 to 4.0

Tensile strength: 30-35 kg./sq. mm.

-140 kg./sq. mm.

The shaped bearing members may be subjected to an Brinell hardnessnumber:

1C6 Patented Aug. 18, 1959- The explanation found In high-aluminumalloys,-

3 after treatmentto improve the properties of the bearing alloy. Thataftertreatment is performed, e .g byheat: ing the shaped parts to atemperature of 100-280 deg. C. The duration of the heat treatment is atleast three hours, preferably 24-48. hours. The. efiectachieved by thevaftertreatment is surprising.

"Whereas Without heat. treatment the elongation is. The

3..6%, itincreases after the treatment to 10%. strength and. hardnessremain substantially unchanged.

1 The impr'ovemeri'Lofthe'properties by the aftertreatmentis'expla'ined. by a change in internal structure. In therange'ofconcentrationsstated the zinc-aluminumcopper alloys consist ofjaternary phase andaluminumzinc mjxedwcrystal. The treatment according tothe invention leads to. a partly euteetoid disintegration of the,

Fr oni an alloy containing 35% aluminum, 7.5% cop- P9 11. a ance.mmercia p re i a bearing sleeve is magleby casting andheat-treated at250 deg. C. for

24 h9urs. Thestrengthis 32 kg./sq. mm, the elongation 19%.. h Brins brdness n mbe 120 a q- When the hearing was loaded with 50 kg./sq. cm. a

ternperature rise of 60 deg, C. was observed.

4 Example 2 From an alloy containing aluminum, 8% copper, and thebalance of commercially pure zinc, a bearing sleeve is made by castingand is heat-treated at 250 deg. C. for 24- hours. elongation. 5%, theBrinell hardness number.l0 8 kg./sq. mm. When'the bearing .Was loadedwitli kg.'/sq. cm.,

a maximum temperatufe'rise of deg'. C. was observed.

1. A method of producing an aluminumrzincfiopper alloy for bearingpurposes comprising thejsteps of: 'producing a compositionalloy of. 35to 50% Al 5.8.to 8.3% Cu, 0 to 0;3'%"1=e,"oto;0.2s j% s1, and 59.2"to41.15% of Zn; of shaping said' alfoy; and of heat-treating said alloy attemperatufes from 10.0. to 280?. C.- fora time fro'm 24 to 48 hours.

2. A method of producing an aluminum-zinc-copper alloy comprisingthesteps of ,producingan alloy compo sitionloff35fto'50% iii/83116 1f2'i5'%Cu; 0 in 0.3%- FejO'to 0.25% Si, and 56.2 to 36.95% of'Zn,andheattreating said alloy attemperatures from for a perio'd'from 24 to.48 hours. I

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES 1948 Meta1s I-Iandbook, page 1244.

The strehgthis 32.5 kg./sq. mm. the

' to 280 C;

1. A METHOD OF PRODUCING AN ALUMINIUM-ZINC-COPPER ALLOY FOR BEARINGPURPOSES COMPRISING THE STEPS OF: PRODUCING A COMPOSITION ALLOY OF 35 TO50% AL, 5.8 TO 8.3% CU, 0 TO 0.3% FE, 0 TO 0.25% SI, AND 59.2 TO 41.15%OF ZN; OF SHAPING SAID ALOY, AND OF HEAT-TREATING SAID ALLOY ATTEMPERATURE FROM 100 TO 280*C. FOR A TIME FROM 24 TO 48 HOURS.